"Besides Earth itself, Mars is the most-studied planet in the solar system. One reason for the abundance of probes and landers, of course, is that Mars is relatively close. Another is that Mars appears to have once had plenty of liquid water on its surface. And where there is water, astrobiologists whisper about the possibility of life.
But Mars and Earth are not the only places in the solar system that either have, or have had, water. On October 14th a NASA probe called Europa Clipper blasted off from Florida. As its name suggests, the mission’s target is Europa, one of the biggest of Jupiter’s 95 known moons.
Europa is a snowball slightly smaller than Earth’s moon. It has an atmosphere that is thin to the point of non-existence, a crust of water ice and a surface temperature of around -180°C. But scientists think a vast ocean exists beneath the ice, kept liquid by friction produced as Europa is kneaded by Jupiter’s powerful gravity.
Over the past few decades scientists have become steadily more excited about the life-bearing potential of such “icy moons”.
Besides Europa, these include Ganymede and Callisto, two other Jovian moons; Enceladus, which orbits Saturn; and Triton, the biggest satellite of Neptune.
Europa’s icy crust is thought to be tens of kilometres thick. Europa Clipper will, therefore, not be able to tell whether there actually are any aliens swimming around in the depths. Instead, its job is to assess whether the moon is the sort of place where life might plausibly arise. One of the probe’s tasks will be to characterise the size and saltiness of the ocean. NASA’s present best guess is that it varies from 60km to 150km deep. If that is right, then, despite its small size, Europa would have about twice as much liquid water as Earth does.
But although water is thought to be extremely useful (and possibly even vital) to the development of life, it is not enough on its own. To qualify as habitable, a world needs enough other elements to allow complex chemistry. Besides the hydrogen and oxygen in water, a common shortlist adds carbon, nitrogen, phosphorus and sulphur. All of these have already been found on a different icy moon—Enceladus.
On Enceladus, plumes of ocean water jet out into space through cracks in the crust. In 2008 Cassini, another American spacecraft, flew straight through one of those plumes, discovering six of the elements on the astrobiological list.
Whether such plumes exist on Europa is an open question, says Robert Pappalardo, Europa Clipper’s chief scientist. Europa’s ice shell is much thicker than Enceladus’s, he says, which makes it less likely that surface cracks or fissures would reach all the way to the ocean. Some tantalising—but uncertain—images from telescopes nevertheless show things that look plume-like. But follow-up observations with the space-based James Webb Space Telescope have so far failed to spot any.
If plumes do not exist, then Europa Clipper will have to content itself with examining the moon’s surface. That surface is notably smooth and relatively free from impact craters, which suggests it is regularly renewed by processes a bit like plate tectonics on Earth. That, in turn, suggests that chemicals that form on Europa’s surface might have a way down to the ocean, and vice versa. Studying Europa’s surface may, therefore, give valuable clues as to what lies beneath.
The final ingredient for a habitable world is a source of energy for life to exploit. Whatever that might be on Europa—far from the Sun, and beneath kilometres of ice—it will not be sunlight. That is a bit of a problem. On Earth almost every living thing ultimately depends on photosynthesis for its energy, including the rich ecosystems in the ocean depths, discovered in the 1980s and which helped the idea of life on Europa gain a foothold. Their inhabitants do not benefit from sunlight directly, but their metabolisms are powered by chemicals created in the photosynthesising, oxygen-rich surface oceans far above.
There’s none of that on Europa.
But there does not have to be. Some of the microbes living in fissures in the earth’s sea bed make use of chemicals that come entirely from below, rather than above. It is a scant source of energy, but a real one. And Europa might offer much the same. Analysing Europa’s surface chemistry may give clues as to whether something similar could, at least in principle, be happening on its ocean floor.
Two come along at once
And Europa Clipper will not be the only probe hanging out at Jupiter. Last year saw the launch of a European probe called the Jupiter Icy Moons Explorer (JUICE). It will likewise examine Europa, as well as Callisto and Ganymede, two other moons that are also thought to have oceans. The vagaries of orbital mechanics mean that, despite its later departure, Europa Clipper will arrive in 2030, a year before JUICE.
If the findings from the two missions are sufficiently exciting, then the next step could be to send a lander. Scouting for landing sites on Europa is another of Europa Clipper’s goals. But the probe will not be able to build a perfect map of the moon’s surface.
Jupiter’s powerful magnetic field produces areas of intense radiation near the planet, enough to fry any spacecraft that lingers too long. Europa Clipper will, instead, make 49 looping flybys, gathering as much data as possible each time before retreating to a safe distance. The world’s alien-hunters will be hoping it survives." [1]
1. Waterworld. The Economist; London Vol. 453, Iss. 9419, (Oct 19, 2024): 77.
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